1. Field of the Invention.
The present invention relates to the formation of threaded fasteners and particularly to the generation of rolled form helical threads. More specifically, this invention is directed to tooling for forming seam-free threads in fastener blanks and especially to dies having a unique thread profile geometry. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
The formation of a threaded fastener by subjecting a generally cylindrically-shaped portion of a preformed metal blank to a thread-rolling process is well known and will not be described in detail herein. It should suffice to note that a widely used technique, which enables quality threaded fasteners to be produced at high production rates, involves serial feeding of the preformed fastener blanks into the gap between a pair of "flat" thread-rolling dies. By imparting planar motion to one of the flat dies relative to the other, a blank disposed between the dies will be caused to roll as it traverses the length of the stationary die. During the rolling of the blank, because of the forces applied thereto by the dies, the metal comprising the blank will flow so as to form a thread having a shape determined by the profile of the lands and grooves machined in the opposing faces of the dies.
Fasteners having conventional rolled threads, fasteners formed in accordance with the above-briefly described prior art technique which employs a pair of flat thread-rolling dies for example, are suitable for most applications. It is to be noted, however, that the generation of rolled form helical threads will typically produce a thread characterized by a crest seam or seams, i.e., a fissure at the crest of the thread. In accordance with SAE specification AS7456, issued Feb. 19, 1991, seams at the roots of a thread or in the flanks of a thread below the "pitch line" or midpoint of the thread depth are generally not permissible, but a seam in the thread crest, which is sometimes referred to as a crest lap or crest crater, may be tolerated if not of excessive size. However, in many applications, especially where cleanliness is critical, a crest seam provides a potential haven for bacteria and other contaminants. Accordingly, a thread-rolling technique and apparatus which would produce a seam-free thread without a significant reduction in obtainable production rates has been long desired.
It is to be noted that previous attempts to meet the above-mentioned long standing desire for a mass-produced seam-free rolled helical thread have encountered numerous obstacles. The most prevalent of these obstacles, where the use of flat thread-rolling die geometry is the technique of choice, has been slippage of the blank being worked during transfer of the thread form from the die to the workpiece as one of the two cooperating dies rotates the workpiece. Any such slippage will result in the production of a defective thread.